Rival: Battle for Azeroth Season 1

Achievement

Rating required in any rated pvp format

Combatant: Battle for Azeroth Season 1

1400

Challenger: Battle for Azeroth Season 1

1600

Rival: Battle for Azeroth Season 1

1800

Duelist: Battle for Azeroth Season 1

2100

the following achievements are exclusive to 3v3

Gladiator: Battle for Azeroth Season 1

50 games in 3v3 while above 2400 rating

Dread Gladiator: Battle for Azeroth Season 1

Top 0.1% of the 3v3 ladder with 150 games
played in your current faction

This achievement is marked differently from the previous Rival achievements, which had a description of players requiring to be in a certain percentage before the season ended, and a certain number of wins.

For example, Rival was rewarded in a season <BFA if you were in the top 3-10% of arena participants, and had 50 3v3 wins for that season. With the upcoming rating overhaul in BFA, this will be different. Players are now placed in a bracket based on their rating. At the time of this comment, there has been no information if titles from these brackets are going to be permanent or temporary (which would follow suit with previous arena season patterns).

• 
  
• Combatant: 1400–1599
• 
  
• Challenger: 1600–1799
• 
  
• Rival: 1800–2099
• 
  
• Duelist: 2100–2399
• 
  
• Gladiator: 2400
• 
  

Quoting the article, they are still working out how to distribute end of season rewards (which includes the titles listed for the brackets above). This means that titles underneath the Rank 1 Gladiator are still being worked out, and according to the article:

We’re still in the process of working out the possible end-of-season rewards, but for now we can say we’ll be keeping the special seasonal Gladiator titles for the very top players at the end of each season. Those who get the Gladiator title will also need to win an additional number of games to earn the mount as their reward.

Here is the official rating update posted by Blizzard.